This invention relates to inkjet-printing apparatus, and in particular to apparatus for cleaning, maintaining and supplying ink to a printhead of an ink jet printer.
Types of printers exist which make use of so called piezo-electric ink-jet printing technology. A piezo-electric printhead (also sometime referred to as PZT printhead) of such a printer is characterised in that it has a plurality of miniature jetting chambers or “jets” closely arranged in an array. Each jet is arranged to project ink from a respective one of an array of orifices defined by the printhead, and the jets are selectively energised by a controller to project (or not project as the case may be) “pixels” of ink. The ink is projected onto a substrate to be printed, relative movement between the printhead and the substrate resulting in ink projected from one orifice being deposited along an elongate path on the substrate. The printhead is arranged within the printer such that the array of jets extends at a predetermined angle (for example 90°) to the direction of the relative motion between the printhead and the substrate. The controller selects a first combination of jets through which ink is projected, and then a second combination of jets, etc, relative movement between the substrate and the printhead resulting in a two dimensional image being printed on the substrate.
Piezo-electric ink jet printers are used in many office and industrial applications. Industrial printing applications include packaging printers, which print directly onto substrates such as cardboard boxes, trays, flexible film and labels.
Current industrial printers use a range of different inks in conjunction with a variety of different models of piezo-electric printheads. Some utilise liquid ink, others utilise solid ink which is heated so as to phase-change to liquid within the printer, the ink being liquid at the time it is ejected from the jets.
Printhead arrays used in industrial applications typically range in length from 10 mm to 70 mm. In high resolution printheads, as many as 512 jets are arranged in a 70 mm long linear array on a printhead. It will therefore be appreciated that the jets are very small. Each jet ejects a very small (picoliter order of magnitude) droplet of ink each time it is energised by the controller.
Whilst such printers can produce fine resolution images, they are subject to image degradation due to one or more of the jets becoming blocked by dust or other contaminant, or otherwise fail as the result of a jet not being filled with ink to adjacent the orifice at the time the ink is to be projected (commonly referred to as a “de-primed” state), so that they no longer eject ink properly. This jet failure (often referred to as “jet drop out”) results in a degraded image having one or several unprinted lines running throughout the printed image in the direction of relative movement between the printhead and the substrate. A contaminant that does not completely block a jet can also cause problems by causing the deflection of a droplet of ink as it is ejected from the jet, such deflection resulting in a distorted image.
In certain industrial applications such as printing onto cardboard boxes or trays, a significant contributor of contamination is dust, either airborne or resting on the surface of the cardboard. The piezo-electric printing process requires the printhead jet array face to come into very close proximity with the target substrate. Furthermore there can be a build up of static electricity in the production process in the area around the printer, which can result in dust being positively attracted to the print face. Thus particularly in industrial applications contamination of jets is a major problem.
Given the problems referred to above, it is common practice, particularly in industrial applications, to follow a procedure often referred to as “purging” in order to maintain or restore the proper function of the jets in the printhead. The purging procedure involves forcing ink through all the jets of the printhead, with the intention of flushing out and removing any contaminants from the jets and ensuring that the jets are correctly primed, that is filled with ink to adjacent the orfices. After performance of the purging procedure, the jets should be in a stable working condition (commonly referred to as a “primed” state) such that high quality printing can be commenced or recommenced.
In known industrial printers, the purging procedure is typically initiated by the user. The purging action is typically brought about by putting an ink supply to the printhead under sufficient positive pressure to force some ink through all the jets of the printhead. This can be achieved by the user applying pressure manually to an ink reservoir communicating with ink in the printhead, or by applying pressure using a pump to the reservoir. Typically, positive pressure is applied for a period of a few seconds.
In industrial applications, such as printing onto cardboard boxes or trays, the printhead is often mounted adjacent a production line, so that the boxes pass in close proximity to the printhead. In these applications the printhead is most often mounted such the jet array is vertical, or at some predetermined angle to the horizontal. When ink is purged through the jets, it collects on an orifice plate of the printhead in which the jet orifices are formed, and begins to flow down the orifice plate under gravity. The flow is of ink is relatively slow, the ink taking a number of seconds to flow down to the bottom of the printhead. Of course, the longer the printhead in the vertical direction, the longer the downward flow of ink takes.
Since a relatively large volume of ink (as compared with the volume of ink normally projected by an individual jet) is purged out of the orifices, the jets are not able to jet ink properly to create an image until substantially all the ink has flowed down away from the jets or has otherwise been moved away from the jets. Generally steps are taken to remove purged ink. For example, purged ink is soaked into a cloth which is then disposed off. This can easily result in ink being dripped onto the floor or surrounding factory equipment, creating unwanted mess, which is difficult to remove and clean. The wasted purged ink is a considerable percentage of the total ink consumption. Furthermore, wiping away purged ink in this way typically takes several seconds. The total time taken to complete a single ink purge procedure is thus substantial.
Accordingly, although the purge procedure is generally successful in recovering jets, it can cause a number of problems. Firstly the purge procedure generally requires manual intervention and takes at least several seconds. It will be appreciated that the printhead cannot properly print images whilst it is being purged. Therefore it is therefore often necessary to stop an associated process (for example a production line) until the purge procedure has been completed. Secondly, the amount of ink consumed in the process is relatively large, often being several times greater than the amount of ink used to print an image.
Various proposals for improving ink purge procedures have been proposed. For example, one known purging procedure described in British Patent GB 2339170 provides an aperture in a plate defining the orifices from which ink is projected, the aperture being positioned in use vertically beneath the jets. Either in response to detection of jet dropout requiring remedial treatment, or at intervals for maintenance purposes before jet dropout has occurred, an ink feed reservoir is pressurised with a pump or manually to cause ink to flow out of the printhead orifices. After purging, ink flows down the orifice plate under gravity towards the aperture. The ink is sucked through the aperture and ducted to a reservoir for filtering and subsequent re-use. Ink wastage is thus reduced, but as the ink flows only under gravity it will be appreciated that it takes at least several seconds for the ink to flow away from the jets to the aperture. Even on production lines operating at modest speed the time taken for purged ink to be removed from the orifice plate is too long for purging to be carried out between two successive product print cycles. Thus it will be appreciated that this known purging procedure takes too long to avoid the necessity of stopping the production line whilst purging occurs.
International patent specification WO 89/04255 describes an ink jet priming system in which pressurised air is used to purge ink into an ink collection system which is located adjacent ink jets to be purged only during the purging procedure. During purging, a sufficiently high pressure is applied to the ink such that it streams into the collection system rather than trickling out of the jets. Thus purged ink is removed from adjacent the jets without requiring manual intervention, but only as the result of providing a complex mechanical assembly to achieve the necessary relative movement between the jets and the ink collection system. The described system has the capability of controlling the shape of a pressure pulse that is used to purge ink from the jets, that pulse having a “soft” profile (slowly rising leading edge and rapidly falling trailing edge) or a “hard” profile (rapidly rising leading edge and rapidly falling trailing edge). In the described examples, the slowly rising leading edge part of the pulse has a duration of 100 milliseconds or 200 milliseconds. It is stated that it is desirable to abruptly terminate the supply of air which causes the ink to flow out of the jets (the rapidly falling trailing edge of the pulses). This is said to cause the streaming of ink from the jets to abruptly cease. It is stated that the described arrangement eliminates the trickling of ink from the jets.
European Patent Specification EP 1016530 describes another arrangement for cleaning and preventing obstruction of the orifice plate of an ink jet printer. The device described incorporates a cleaning assembly which is moveable relative to the orifice plate to a position in which a closed chamber is formed over the orifice plate. A fluid is then pumped through the closed chamber so as to clean any contaminants from the orifice plate surface or any individual orfice in the plates.
The highly complex arrangement of EP 1016530 is presented as an improvement of an earlier proposal which is described in U.S. Pat. No. 4,970,535. That US patent discloses an arrangement in which an ink jet orifice plate cleaner is moved into engagement with the orifice plate so as to provide a closed air passageway through which air is directed to provide the required cleaning effect. Once again the described assembly is complex, requiring relative movement between a “ready to print” configuration and a “cleaning” configuration. Furthermore it is stated in EP 1016530 that air cleaning in the manner suggested in U.S. Pat. No. 4,970,535 does not provide acceptable results.
European patent specification EP 604029 describes another cleaning arrangement in which an air curtain is directed across an ink jet orifice plate. The jets are located behind an aperture in the plate, projected ink passing through that aperture before being deposited upon a substrate to be printed. Air flows between the apertured plate and the orifice plate and thus across the orifices. The airflow is maintained during printing at an airflow rate that is sufficiently slow as to not significantly effect ink projection. The purpose of this airflow is to avoid the build up of contaminants on the orifice plate, not to clean ink off the orifice plate. Cleaning of the printing head requires a separate operation, involving release of a latch mechanism to enable the orifice plate to swing away from a “ready to print” position to a position in which it can be readily cleaned. Positioning the orifice plate behind another plate means that none of the orifice plate is open to the space through which the substrate to be printed travels relative to the orifice plate. As a result the distance through which ink must be projected is relatively large.
U.S. Pat. No. 5,184,147 describes an inkjet printhead maintenance system incorporating various relatively moveable components including a slowly moving elongate mechanical wiper and an “air knife”. The air knife generates a narrow stream of air which is swept across the orifice plate. Thus the air knife requires relative movement between the structure generating the narrow stream of air and the orifice plate. Furthermore, the air knife is provided as only one of a number of complimentary cleaning mechanisms in an overall assembly of great complexity.
In the absence of a simple fast operating jet purging system, in many applications purging generally requires the stopping of an associated process, for example a production line. As a result, users prefer to initiate the purge procedure as seldom as possible. This can result in a compromise between production line efficiently on the one hand and image quality on the other. In practice, it is common for operators to wait for printing quality to deteriorate significantly before initiating the purge procedure. It will also be appreciated that the the longer dust and debris is allowed to build up on the printhead, the more purging and cleaning of the jets is likely to be necessary to fully restore print quality.